Control apparatus for purged and pressurized electrical equipment has been developed for protection against explosions from electric sparks in an environment containing explosive vapors, gases or dusts.
A National Fire Protection Association standard, i.e., NFPA 496, outlines the requirements for purged and pressurized enclosures for electrical equipment. The standard defines types of hazardous locations and types of purging to mitigate or eliminate the hazard.
A Class 1, Division 1 location is one in which ignitable concentrations of flammable gases or vapors exist under normal operating conditions; or in which ignitable concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage; or in which breakdown or faulty operations of equipment or processes might release ignitable concentrations of flammable gases or vapors and might also cause simultaneous failure of electrical equipment.
A Class 1, Division 2 location is one in which volatile flammable liquids or flammable gases or vapors are handled, processed or used, but in which the liquids, vapors or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems or in the case of abnormal operation of equipment; or in which ignitable concentrations of gases or vapors are normally prevented by positive mechanical ventilation and which might become hazardous through failure or abnormal operation of the ventilating equipment; or that it adjacent to a Class 1, Division 1 location and to which ignitable concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by adequate positive pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided. Electrical conduits and their associated enclosures separated from process fluids by a single seal or barrier shall be classified as a Division 2 location if the outside of the conduit and enclosures is a nonhazardous location.
A Class 2, Division 1 location is one (1) in which combustible dust is in the air under normal operating conditions in quantities sufficient to, produce explosive or ignitable mixtures; or (2) where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitable mixtures to be produced and might also provide a source of ignition through simultaneous failure of electrical equipment, operation of protection devices or from other causes: or (3) in which combustible dusts of in electrically conductive nature may be present.
A Class 2, Division 2 location is one (1) in which combustible dust will not normally be in suspension in the air in quantities sufficient to produce explosive or ignitable mixtures and dust accumulations are normally insufficient to interfere with normal operation of electrical equipment of other apparatus; or (2) in which dust may be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment and dust accumulations resulting therefrom may be ignitable by abnormal operation or failure of electrical equipment or other apparatus.
Under these standards, the following definitions apply. Pressurization is the process of supplying an enclosure with clean air or an inert gas with or without continuous flow at sufficient pressure to prevent the entrance of combustible dusts and refers to Class 2 applications. Purging refers to Class 1 applications and is the process of supplying an enclosure with clean air or an inert gas at sufficient flow and positive pressure to reduce to an acceptably safe level the concentration of any flammable gas or vapor initially present and to maintain this safe level by positive pressure with or without continuous flow. Type X purging reduces the classification within an enclosure from Division 1 to nonhazardous. Type Y purging reduces the classification within an enclosure from Division 1 to Division 2. Type Z purging reduces the classification within an enclosure from Division 2 to nonhazardous.
The following U.S. Pat. Nos. are representative of pressurization and purging systems previously disclosed for use with various types of electrical equipment;
Williamson U.S. Pat. No. 2,449,436 discloses a system for pressurizing an apparatus containing volatile liquids with carbon dioxide and condensing the vapor from the pressurizing gas.
Bernard et al U.S. Pat. No. 2,467,181 discloses a system for pressurizing and purging an electric meter
Thayer et al U.S. Pat. No. 2,807,755 discloses a system for pressurizing an electric switch box.
Pomon U.S. Pat. No. 2,816,678 discloses a system for pressurizing an electric panel board to render it explosion-proof.
Kleinpeter U.S. Pat. No. 2,985,337 discloses a system for pressurizing an electric enclosure having porous plates covering the discharge openings from the enclosure.
Rickert et al U.S. Pat. No. 4,211,251 discloses an automatically controlled system for pressurizing an electrical apparatus to protect it from explosive gases or vapors.
Volker et al U.S. Pat. No. 4,537,043 discloses a system for pressurizing an electrical apparatus with nitrogen and carbon dioxide to protect it from explosive gases or vapors.
Hsieh U.S. Pat. No. 4,616,694 discloses a system for controlling a ventilating system to protect electronic equipment from explosive gases or vapors.
Williams U.S. Pat. No. 4,802,502 discloses an automatically controlled system for pressurizing an electrical apparatus to protect it from explosive gases or vapors.